Structural articles and method of making

ABSTRACT

CERAMIC ARTICLES OF GENERALLY HONEYCOMB STRUCTURE ESSENTIALLY FREE OF ANY DEMARCATION LINE AT NODES ARE PROVIDED TOGETHER WITH PROCESSES FOR FORMING SUCH ARTICLES BY FIRING GREEN STRUCTURES HAVING TEMPORARY BONDS AT NODES WHERE SHEETS ARE IN CONTACT.

p 1973 J. R. JOHNSON STRUCTURAL ARTICLES AND METHOD OF MAKING OriginalFiled May 2, 1960 United States Patent Int. Cl. F28f 3/00 U.S. Cl.165-166 25 Claims Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE Ceramic articles of generally honeycombstructure essentially free of any demarcation line at nodes are providedtogether with processes for forming such articles by firing greenstructures having temporary bonds at nodes where sheets are in contact.

This application is a reissue application of my application Ser. No.331,649, filed Dec. 17, 1963 and issued as U.S. Pat. No. 3,444,925 onMay 20, 1969, said application being a continuation of my applicationSer. No. 26,372, filed May 2, 1960, now abandoned, which application isa continuation-in-part of my now-abandoned ap plication Ser. No.746,263, filed July 2, 1958, the latter being a continuation-in-part ofmy application Ser. No. 657,503, filed May 7, 1957, now Patent No.3,079,273.

This invention relates to heat-resistant thermallyrigidified complexshapes and structural articles, and to methods for producing the same.More particularly, the invention is directed to rigid, sturdy,non-burnable, heatunified, refractory, high-surface-area structuralcomponents having, as an essential part thereof, thin corrugated webs orfilms, and means for making such structural components.

Following the teachings of this invention, it is possible to fabricateceramic heat exchangers of relatively minute size with a multitude ofthin-walled passages which can withstand extraordinary high temperatureswithout corrosion or erosion due to the effects of high temperaturefluid media (e.g., gases or liquids). Heretofore, insofar, as is known,sintcrcd ceramic heat exchangers have been of relatively enormous sizeand thickness (e.g., brick recuperators or regenerators of glass tankfurnaces), highly inefficient as heat exchangers largely because oftheir bulk, and have functioned primarily as heat storage or capacityelements. Heat-unified intricately-designed corrugated ceramic heatexchangers of this invention, on the other hand, permit rapid andeflicicnt heat exchange, with controlled fluid media flow, as desired.In comparison with prior art heat-unified bodies of equal over-alldimensions and similar composition, bodies hereof are made extremelylight in weight, and are many times more efiircient as heat exchangers.In effect, the new order of design possibilities introduced into theceramic art by this invention permits the formation of heat exchangersof comparatively minute size which function as efficiently o; moreefficiently than the bulky types heretofore availa lo.

In many industrial establishments much heat is carried by waste fumesout into the atmosphere and is wasted.

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Some attempt has been made to capture such heat by passing fumes throughheat exchanges formed of ductile metals, but of necessity such metalheat exchangers have been placed a distance from the source ofcombustion so as to prevent their destruction by the intense heat orcorrosive atmospheres of combustion. Thus, their use has not permittedmaximum capture of the heat from exhaust or waste fumes. Heat-unifiedceramic heat exchangers of this invention may be placed in or adjacentto most sources of combustion common in industry, therefore permittingmaximum extraction and utilization of heat from waste fumes; andefiicient, highly responsive heat exchange is accomplished using mythin-walled intricate articles of corrugated web design.

A further illustration of a long-sought-after article, now realized bythis invention, is a multipassaged, thinwebbed, heat-resistant supportstructure for catalysts. A structure of this type for use in catalyticoxidation of exhaust fumes from automobiles is an example. Such astructure should have a multitude of small passages of high surfacearea, be compact and relatively light in weight, and further should becapable of operation at the intense temperatures that exhaust fumes aredischarged from a cylinder block without suffering undue corrosion.Heretoforc, integrated or unified ceramic catalytic oxidizers made byconventional methods have been very bulky and have had an extremely lowsuperficial or geometric surface area-to-volume (or mass) ratio. Whilemetal structures formed of thin corrugated metal sheets satisfy many ofthe requirements aforedelineated, for oxidizers, they do not possess thenecessary resistance to the corrosive effects of hot exhaust fumes, andare generally unsuitable as catalyst supports. Following the teaching ofthis invention, light-weight articles of high geometric surface-area andlow volume may be fabricated so as to possess such required resistanceto heat and corrosion.

This invention now makes possible light-weight, heatresistant turbineblades, light-weight nuclear fuel elements and structures, as well as ahost of other highly heatresistant, light-weight structural articles ofcomplex shape, e.g., corrugated outer shells for missiles to provideinsulation and cooling passages, corrugated burner blocks for radiantgas burners, corrugated tubes (annular or longitudinal corrugations),etc. For the first time, insofar as I am aware, light-weight, verythin-walled, multiplewebbed, thermally-rigidified, complex, corrugated,nonburnable structures made from non-ductile, non-malleable, sinterablcmaterials, with positive and secure bonds or structural joints betweenparts, are now possible. Structural joints between parts of the complexstructures hereof are conveniently formed so as to be indistinguishablefrom the structure of thin-walled corrugated parts of the end article,which offers considerable advantage from the standpoint of total heatresistance of the end article as distinguished from weakness oftemperature resistance such as those created by brazing corrugatedductile metal parts together. Thin-walled corrugated structures, whileeasily formed out of malleable and ductile metals and the like, have notheretofore been available out of the brittle materials such as, forexample, ceramics. In fact, the nature of ceramic materials militatesagainst the possibility of using them to fabricate visual duplicates ofthin corrugated metal structures.

As aforcnotcd, ceramics are brittle, whereas ordinary metals are ductileand malleable. A ceramic cannot be molded after firing, whereasmalleable and ductile metals are easily shaped. Ceramics are known tomaintain their strength at high temperatures, whereas heating of commonmetal objects to ceramic firing temperatures ordinarily causes them tocorrode or melt or deform out of shape. In dealing with some commonmetals of relatively low melting point, forming can be accomplishedunder the heat conditions needed for fluidization or softening of themetal, and an article of desired shape or configuration can be formeddirectly in a die, whereas in the ceramic or refractory field, formingof the desired shape or configuration must generally be followed by afiring step to rigidify it to a hard and permanent state. Firing to thepermanent hard and rigid state (e.g., the ceramic state) frequentlyinvolves shrinkage of the article, which is not a problem in shapingsolid ductile and malleable metals.

Shrinkage of a ceramic during firing creates serious problems for theceramic manufacturer. It is a problem in making extremely thin flatelements such as capacitors, since shrinkage is frequently accompaniedby warpage. Thus, the firing of flat thin elements for capacitors hasbeen accomplished between flat refractory plates, usually with aplurality of flat capacitor elements in a stack between such refractoryplates, so as to prevent edge curling and warping of the capacitorelements during the sintering operation.

To some extent, the manufacture of such thin ceramic pieces forcapacitors and similar end uses has been enhanced by the use oftemporary organic binders to facilitate the formation of thin films ofgreen ceramic for firing; but such has not eliminated need for controlof shrinkage, warpage and curling in the firing operation.

In the firing of a thin film of organically-bonded or plasticizedsinterable particles, the first significant loss of film strength occurswhen the fugitive or temporary organic binder or carrier is volatilized,usually at a temperature below but up to about 500 C. At such point inthe firing cycle the film is extremely fragile. The fragility of a thinfilm (such as hereinafter illustrated in the examples) from which theorganic binder has been volatilized can be demonstrated by simplylighting a match to a sample of the film to burn out the organic binder.The resulting burned film is so fragile that it tends to flake and fallinto minute particles when touched with a pencil. Not until subsequentheat-unification (e.g., as by sintering, which amounts toheat-unification of a particulate body into a hard, rigid permanentstate by interfacial bonding between particles of which it is composed,without actual melting of the mass of particles themselves) isaccomplished at relatively high temperatures (e.g., 1000 C. or more, formost ceramics) does the film again gain strength, and its strength aftersintering is of quite a different type than that exhibited in theoriginal film with organic binder present. Thus, it will readily beappreciated that the firing of a plurality of thin fragile films andwebs in compound-curved shape, stacked on top of each other with amultitude of interspersed passages and structural configurations as wellas joints to be preserved, is by no means to be expected to produce acoherent composite article of sound, predictable, structural featuresand integrity, with the pre-formed structural configuration preserved inthe final article. Indeed, experience would indicate to the contrary,since unfired or green thin ceramic rods have been noted to sag whenfired in a horizontal position with only the ends thereof supported.Surprisingly, however, I have discovered amazing preservation ofstructural features during firing of the shaped green corrugatedarticles hereof, and a few flexibility in the step of firing sucharticles.

It may be noted at this point that I am familiar with Cohen Patent No.2,552,937, which sets forth that a ceramic heat exchanger may be formedby winding in overlapping convolutions upon itself a strip of materialflat on one side and having projecting spacer vanes on the other side.The strip illustrated by Cohen is not corrugated as taught herein, andis rather massive in structure, with relatively thick spacer vanes andflat sections. The spacer vanes are necessarily thick in order to beformed as taught in the patent; and further must be relatively thick, inview of their straight projecting design, in order to possess therequired strength in the green state for a spacer function. Also, noprovision is made in the Cohen patent for positive bonding of theabutting projections to the flat side of the strip material; and theonly apparent means for accomplishing a fired bond between such abuttingsurfaces in the patent appears to be that which might arise incidentallyin the step of firing itself. Generally mere abutting surfaces of greenceramic do not bond together during the step of firing, as may readilybe appreciated when it is realized that capacitor discs are frequentlyfired in stacked abutting relationship between refractory setter plateswithout interbonding between the capacitor discs. While scatteredincidental bond areas arising during firing may be suitable for someregenerative heat exchangers, where gaseous escape from one chamber toanother may not be harmful, they would be entirely unsuitable in acountercurrent or counterflow heat exhanger where the flow of one fluidmedia is to be maintained entirely separate from a different fluid mediareceiving or giving up heat to the other through the walls of theheat-exchanger chambers. Of course, relatively massive ceramic wallthicknesses further militate against satisfactory operation of devicessuch as counterflow heat exchangers where elficient and highlyresponsive heat exchange is required as temperature differentials changeduring operation.

Advantageously, compound-curved thin-webbed intricately-designedarticles such as illustrated herein provide the art with newlight-weight refractory articles highly versatile in use applications,retaining many of the advantages of articles formed by brazing intricatedesigns of ductile and malleable metal Webs together, while at the sametime avoiding the disadvantages of brazed metal Web articles as well asthe limitations of ceramic design and fabrication as taught in the priorart. As aforestated, the thin-webbed articles hereof surprisingly retaintheir structural configuration during firing even though past experiencewith thin green ceramic webs would indicate that warpage would beessentially uncontrollable in the absence of firing the webs betweenrefractory surfaces holding the webs in the shape of the final desiredarticle.

Structural aspects and features of the sintered articles of thisinvention will be described and illustrated by ref erence to a drawing,made a part hereof, wherein:

FIGURE 1 is a perspective view of a portion of a crosscurrent-flow heatexchanger having corrugated ceramictype films between thin sheetmembers;

FIGURE 2 is a perspective view of a canister or cartridge article havinga spirally wound corrugation formed of sinterable material;

FIGURE 3 is a perspective view of a turbine blade with a portion of theouter skin cover broken away to show reinforcement b-y corrugated filmsformed of sinterable material;

FIGURE 4 is a perspective view of a sintered laminate of corrugatedfilms between thin fiat sheets; and

FIGURE 5 is an enlargement of the encircled portion of FIGURE 4.

Referring to FIGURE 1, the portion of a cross-current heat exchangerthere shown comprises a laminate of three thin corrugated films oflayers 10, 11 and 12, separated from each other by two thin flat sheetmembers 13 and 14, and positioned between outer sheet wall members 15and 16. The corrugated films are welded to adjacent sheet members alonglines of contact between the sheet members and longitudinal ridges ofthe corrugations of the films. Corrugations of corrugated film 12 runperpendicularly to the corrugations of corrugated films 10 and 11.

In operation, a fluid medium of one type or temperature is passedthrough the channels formed between the corrugations of corrugated film12 and sheet members 13 and 14, as indicated by arrow A, Whereas a fluidmedium of a different type or temperature is passed cross-wise to theflow of the first fluid medium and through the channels formed by thecorrugations of corrugated film 10 and sheet members 15 and 13, as wellas the channels formed by the corrugations of corrugated film 11 andsheet members 14 and 16. Heat transfer from one fluid medium to theother takes place through the extremely thin walls of the channels inthe article, permitting highly efficient heat exchange. Of course, theheat exchange properties and thermal resistance of an article of thisinvention may be varied by using different sinterable particles fromwhich to fabricate the article.

While only one conduit connection 17 with the heat exchanger laminae ofFIGURE 1 is illustrated, such conduit 17 being for fluid media flowingthrough channels formed by corrugated film 12 and sheet members 13 and14, it will readily be apparent that a similar conduit connection may beprovided at the opposite end of the channels formed by corrugated film12 and sheet members 13 and 14, so as to channel fluid media as itemerges from such centermost channels of the exchanger. Likewise,conduit connections with the channels formed by corrugated films and 11and sheet members adjacent thereto are provided for channeling theingress and egress of fluid media through such longitudinal channels.Conduit connection 17 as well as other conduits for such a heatexchanger may be formed according to extruding methods well known in theceramic industry, and the extruded conduit welded to the corrugatedassembly according to procedures hereinafter described.

The article illustrated in FIGURE 2 represents a further embodiment ofthis invention wherein a corrugated film 20 and a sheet member 21 are inthe form of a helix, i.e., wound in a spiral to form a helix having anover-all configuration approximating a cylinder. The helix is preferablysinter-welded within a snug fitting ceramic cylinder 22, the latterhaving walls sufficiently thick to protect the helix from cracking orrupture as might otherwise develop out of excessive shock in use. Aceramic ring 23 with flange 24 is fitted (preferably removably fitted)over each end of the ceramic cylinder 22, and packing 25 such as arefractory wool shock-absorbing layer 25 Wrapped around the ceramiccylinder 22 between the end ring members 23. The so-formed assembly isencased within a steel cylindrical jacket 26 with its ends beveled orflanged as at 27 over each end ring member of the assembly to retain thecomponent parts of the article in position.

The cartridge article illustrated is particularly useful where small,light-weight, corrosion-resistant passages of high heat tolerance aredesired, as in catalytic exhaust gas oxidizers, heat exchangersoperating on a revolving cycle, etc. As an illustration of a heatexchanger operating on a revolving cycle, reference is made to suchstructures as those comprising a large cylindrical block memberrotatable on a central shaft through a divider plate above and belowwhich fluid media passes, usually in counter-current direction. Thelarge cylindrical block has a plurality of small cylindrical passagesextending therethrough parallel with the shaft of rotation and each of asize adapted to receive a cartridge member such as here described.During rotation of the large cylindrical block each cartridge membertherein passes sequentially above and below the divider plate; and whileon one side of the plate takes up heat from gasses or fluid mediapassing through its channels, which heat is discharged or released tofluid media passing through the cartridge while the cartridge is in theperiod of its rotation taking it on the other side of the divider plate.

An application of the critical features of this invention as reinforcingmeans permitting, for the first time insofar as I am aware, theformation of heat-resistant, as well as extremely light-weight,structurally-sound articles such as turbine blades is illustrated inFIGURE 3. As there shown, a plurality of ceramic corrugated films 30serve as bracing and reinforcing elements for the skin 31 of a turbineblade. Sheet members 32 are interspersed between the ceramic corrugatedreinforcing elements, preferably sinter-welded to the contacting ridgesof adjacent corrugations, to impart added strength to the light-weightblade. If desired, sheet members 32 may be also be corrugated andadjoining 6 ridges of the corrugated layers sinter-welded. It will beevident that such joining of corrugated sheet members provides ahoneycomb.

As a simplified illustration of corrugated structural features of thisinvention, FIGURES 4 and 5 may be taken as exemplary. The corrugatedfilms 40 of the illustrated structure are separated from each other by,and sinterwelded to, thin plate members 41. The weld zone 42 isillustrated in FIGURE 5, and it will be observed that the zone is barelydetectable, being essentially free of any demarcation line or juncturebetween corrugated films 40 and plates 41, indicating that thecorrugated film and plate are well fused or coalesced together. In manystructures, however, a perfect weld such as illustrated is not essentialfor the attainment of the advantages hereof. As used herein, the termsweld and sinter-weld means fastened together by a thermal-rigidificationstep, including, but not necessarily limited to, the step of sinteringcontacting portions of films or sheets (or even rods) together usingonly materials having the composition of the films or sheets themselves.Welds, of course, may be formed by using intermediate sinterable orfusible ingredients not present in any films or sheets to be joined bysinter-welding, or they may be formed by thermal-rigidificationinvolving infiltration of molten material such as metal betweenparticles in the area of sheet members or bodies to be joined, toconvert the entire area into a rigid permanent bond structure, asillustrated in Example 1 to follow. Preferably, however, the weld areabetween ridges of corrugations and other sheet members, whether thelatter are also corrugated or not, is formed by thermal unification oftemporary bonds formed using constituents or ingredients common to thesheet members to be joined (or common to the surface layer compositionof sheet members to be joined), as illustrated in the examples, so thatthe final fired articles are of monolithic structure in that thematerial of joints (e.g., see FIGURE 5) cannot be separately identifiedfrom the material of the contacting surfaces of the sheet membersthemselves. Such structural articles have bonds equal in thermalstability to the sheet members joined.

It may also be noted here that FIGURE 5 shows a construction useful as anuclear fuel element. The dotted portion represents a uranium compounddispersed sandwich-style in the ceramic corrugations and flat sheets.

Formation of such structures as illustrated in the draw ings, as well asother related structures is readily accomplished according to thefollowing procedure: A plasticized raw material mix containing finelydivided sinterable particles, plasticizing ingredients such as, forexample, organic polymeric resins, and volatile viscosityadjusting mediais formed into a thin film or sheet material by any suitable process,e.g., knife coating, spraying, calendering, extrusion, casting, etc.Such film may be formed as thin as desired, e.g., a mil or so, so longas it possesses suflicient body when free of viscosity adjusting fluidsto retain its integrity after corrugation. Exceedingly thin films,however, are so delicate that they are dilficult to process and handle(but may be useful as permeable membranes), whereas films thicker thanabout Ms inch tend to be too bulky for convenient corrugation as taughtherein. Further, films thicker than inch do not permit the formation ofthe light-weight complex articles discussed herein. Best results areobtained by avoiding extremes; and the advantages of the invention withrespect to strength and structure retention without fragility orfracture problems become particularly apparent when using films about 2to 50 mils thick formed of at least by weight sinterable particles andsuflicient organic polymeric plasticizing ingredients to lend a degreeof flexibility to the film. Advantageously, such thin films in thestructural articles hereof contribute to the thermal shock resistance ofthe fired structures, permitting them to withstand more rapid and severetemperature fluctuations without fracture than could be tolerated bybulk articles of the same material.

In the step of corrugation itself, it is sometimes preferable to supportplasticized green ceramic films on a thin sheet of metal foil, e.g.,aluminum foil, suitably of a thickness on the same order of magnitude asthe film to be corrugated (but usually not greater than about mils), orto sandwich the green ceramic film between two such metal foil sheets asit is passed between the corrugating rolls, suitably at room or elevatedtemperatures. The foil advantageously serves as a carrier to distributecorrugation stresses uniformly, aiding in obviating cracking or ruptureof the green films. Also, in the case of those films plasticized withingredients which impart an elastic memory property to the film, atleast one sheet of metal foil corrugated with the film is desirably leftin position after corrugations for a short time so as to maintaincorrugations in the film and prevent reversion to a flat sheet. Use of afoil of material such as aluminum, however, is not required.Satisfactory results are gained in the absence of a foil; and theabsence of a foil permits temporary bonding of corrugated sheets toother corrugated sheets or flat sheets simultaneously with the step ofcorrugating.

Corrugation of the flexible films may be accomplished using standardcorrugating equipment, and without undue pressure at low temperatures.Mild heating of films or masses to be corrugated, up to elevatedtemperatures not sufficient to burn out all organic plasticizers in thegreen films, has been found useful to impart a degree of permanency tothe corrugations in green films. Usually corrugations of uniformperiodicity are formed, e.g., corrugations of repetitive and uniformwave shape, amplitude, and pitch as illustrated in the drawings; but insome applications, as for example where a corrugated sheet is spiralledupon itself to form a roll or helix, it may be preferable to employ acorrugated sheet having a graduated decrease in pitch, or possibly agraduated increase in amplitude with the pitch remaining the same, orboth. The corrugations most frequently employed are those of standardcurved ridges and grooves; however, other wave shapes of irregularperiodic configuration may be useful, e.g., those representing the sumof all even or odd harmonies, as well as various combinations ofharmonics. In all instances, however, the corrugations are of therepetitive type with both sides of the corrugated sheet exhibiting arepetitive pattern. While small variation from this requirement may notupset end results, and the thicknesses of corrugated sheets may vary atlocations along corrugated wave patterns, all corrugated sheets hereofare of the type having a repetitive corrugated pattern on both sides ofthe sheet; and the term corrugation is to be understood as having thismeaning. The amplitude of corrugations (i.e., the elevation distancebetween the peak of a ridge and the lowermost portion of an adjacentgroove) is at least as great as the thickness of the film that iscorrugated, which means that the elevation distance between the peak ofa ridge on one side of a corrugated film and the peak of a ridge on theopposite side of the film is at least twice as great as the thickness ofthe film itself. Preferably the amplitude is at least twice as great asthe thickness of the film; and the majority of structural articles ofthis invention generally have corrugations with amplitudes at least fivetimes greater than the thickness of the film.

Following corrugation, while the sinterable flexible plasticizedcorrugated films are in the green unfired state, they are sawed, cut andfabricated into assemblies and structures such as illustrated in thedrawings. Where the ridges of corrugations on one side of a corrugatedfilm are to be welded to a sinterable sheet member or panel, the basicraw material mix from which the sinterable film or sheet material wasformed may, diluted with organic solvents or fluids to adjust viscosity,be painted over the ridges of the corrugations as a glue media forafiixing a sinterable sheet member thereto. The solvent of the appliedglue media between the ridges of corrugations and the sheet member maytend to solvate a portion of the adjacent film and sheet member beforevolatilizing into the air. In any event, once the structure is dried, atemporary bond between the ridges and the sheet member is formed, which,after the structure is fired to sintering temperatures, turns into astrong and rigid weld. Alternatively, if desired, heatsealing of panelsheet members to the ridges of contacting corrugations may beaccomplished to form a temporary bond in fabrication.

In the green unfired state, the corrugated structures and assemblies caneasily be cut or sawed to shape, e.g., sawed into over-all shapes asneeded for internal reinforcement of a turbine blade, and bonded to skinpanels, conduits, etc., as desired, using glue media or heat-sealing asaforedescribed.

Where solvent is employed in the bonding operation, the completedstructural article is allowed to dry so as to substantially removevolatile solvents or organic fluids from its joints. Then the structuresare heat unified by using temperatures suitable for sintering of theparticular sinterable ingredients in the corrugated films and otherportions of the structure, as well understood in the ceramic art; orthey are heat-unified by using firing temperatures suitable forinfiltration of molten material between the refractory sinterableparticles in the corrugated films to thereby achieve rigidfication ofthe structure. Heat unification may also be accompanied by a reaction insitu during firing. Illustrative firing conditions for variouscompositions are set forth in the examples hereof. It should be notedthat rapid firing cycles may be employed without damaging the structuralarticles; in fact, shock firing, i.e., firing by placing the article ina furnace held at the temperature required for sintering, followed byremoving it directly to room temperature. is frequently suitable toemploy, particularly with small structures.

My invention will be further described and illustrated in connectionwith the following examples:

Example 1 A plastic mixture is prepared consisting of 39.4 parts ofpolyvinyl butyral (which may be obtained under the trade name Butvar"from the Shawinigan Resin Corporation), 15.8 parts of a polyalkyleneglycol plasticizer, 3.5 parts of Tergitol (a wetting agent sold byCarbide and Carbon Chemicals Company which contains lower alkyl ethersof polyethylene glycol), 33 parts of micronized graphite (particle size2 to 10 microns available from the Dixon Crucible Company), 67 parts of325 mesh coke, 33 parts of 600 mesh silicon carbide and 200 parts of1000 mesh silicon carbide. A sufficient amount of toluene is added tomake the mixture substantiall liquid. Mixing is continued until themixture is substantially homogeneous, and then a thin film is preparedby knifecoating the plastic mixture on a sheet of polymer such aspolyethylene terephthalate to a thickness of about 10 mils. The filmthus prepared is warmed slightly to evaporate the toluene therefrom. Aplasticized sinterable film is obtained containing carbon, siliconcarbide and the resin mixture described. It does not adhere to thepolymer sheet and can easily be stripped olf, yielding the film in asubstantially workable condition to permit the fabrication of structuresas herein described.

The film is stripped off the polymer sheet and sandwiched between twoaluminum foils of approximately the same thickness as the film. Thesandwich is passed through a corrugating apparatus containingcorrugating rolls of a size giving about 7 corrugations of about milsamplitude per inch. Corrugations of the repeating periodic wave typeillustrated in the drawings were used. While in the green state, priorto firing, the corrugated sheet may be shaped, e.g., formed into a tube,spiralled, etc., as may be desired.

Firing of the corrugated film is suitably accomplished by placing itwith a quantity of silicon in a graphite resistance furnace previouslyraised to a temperature of about 3500 F. to 4000 F. and containing ahelium atmosphere, or other inert atmosphere. The quantity of siliconshould weigh about four times the weight of the corrugated film to befired. After a few seconds of heating, the fugitive or temporary organicmaterials disappear leaving a fragile corrugated shape composed of theinitial carbon and silicon carbide in the basic mixture. The siliconmelts and infiltrates the carbon-silicon carbide structure, reactingwith the elemental carbon to form some additional silicon carbide insitu. After about one minute, the corrugated film, heat-unified bymolten metal infiltration between the refractory particles of the bodyas described, is removed from the furnace and allowed to cool to roomtemperature. No control over the rate of cooling need be exercised.(Heat unification of a body of silicon carbide particles by sinteringwould require higher temperatures than illustrated in this example, orspecial lowtemperature bonding additives such as clay.)

The resulting corrugated sheet is useful, for example, as a spacer inhigh temperature nuclear reactor fuel elements. It can withstand longperiods of exposure to temperatures as high as 1300 C. or higher withoutnoticeable deterioration.

More complex corrugated structures and shapes such as illustrated in thedrawings may also be formed using the basic material mix of thisexample. Heat exchangers of various configuration characterized by alaminate as illustrated in FIGURE 1, or a helix as illustrated in FIG-URE 2, are readily fabricated using fiat plasticized sheet members andcorrugated plasticized films of carbon and silicon carbide. Weldsbetween fiat panels and ridges of corrugations, as well as otherelements to be joined, are accomplished by interposing a thin coating ofsolvent diluted basic raw material mix between the ridges ofcorrugations and the panel (or other parts to be joined), allowing ashort period of time for solvent spreading, penetration and evaporation,and then firing as aforedelineated.

Example 2 A basic plastic raw material mix is prepared consisting of 85parts of alumina particles having an average diameter of about microns(with particles varying from about 1 to 44 microns), parts of atetrapolymer, and about 40 parts of a solvent mixture consisting of22.1% ethyl acetate, 38.95% Cellosolve acetate, and 38.95% nitroethane.The tetrapolymer consists of about by weight octadecyl acrylate, 30%acrylonitrile, cyclohexyl acrylate, and 5% acrylic acid copolymerized inethyl acetate. It is in the form of an organosol and sufficientCellosolve acetate (also called ethoxyethyl acetate) and nitroethane areadded to give the solvent mixture aforespecified.

The raw material mix is placed in a porcelain ball mill and milled forabout 8 hours to form a uniform blend of the ingredients. Satisfactoryblends have been formed, however, by merely stirring the ingredientstogether without ball milling. The milled slurry is knife-coated as athin layer on a low adhesion carrier web of polyethylene coated paper toa thickness sufiicient to provide a solventfree coating about 5 milsthick, and the coating is then air dried at room temperature so as to besubstantially free of solvent vehicle.

The dried coated layer is stripped from the low adhesion carrier web ofpolyethylene coated paper, placed on an aluminum foil of about 5 milsthickness, and passed through a corrugating apparatus, the rolls ofwhich are at about 270 F. and provide 15 curved periodic corrugations of30 mils amplitude (i.e., elevation from bottom of valley to top of ridgeof each corrugation) per inch. corrugations of this film under heat ispreferred, as the thermoplastic binder employed, while imparting adegree of room-temperature flexibility to a thin sheet as heredescribed, is relatively stiff as compared to the formulation ofExample 1. The room temperature relative rigidity 10 is desirable forhandling the material in fabricating green shapes.

After corrugating, the exposed ridges of the corrugated strip arefastened to a 5 mil thick fiat strip of plasticized ceramic-particulateof the type above specified in this example by heat-sealing, i.e.,heating the fiat strip by passing it over a heated roll at 270 F. andbringing the fiat strip into contact, using slight pressure, withexposed ridges of the corrugated film while it is still hot from thecorrugating step. The carrier strip of aluminum is then removed from thecorrugated film.

The foregoing structure may be fired by heating it to 1650" C. over aperiod of four hours and then cooling it to room temperature over aperiod of 4 hours, without holding the article any extended time at themaximum temperature. Much shorter firing cycles may be employed; infact, the article can be fired in about 15 minutes by placing it in apre-heated furnace at 1650 C. Cooling can be accomplished essentially asrapidly as desired.

If desired, the foregoing structure while in the green state(particularly where the width of the flat strip and the corrugated filmare the same) may be wound, suitably while it is at room temperature orslightly warmed, in a spiral to form a helix such as illustrated inFIGURE 2, using a coating of the slurry of this example to tack a few ofthe ridges of corrugations to the fiat strip as winding is accomplished.Tacking, however, is not absolutely necessary. In forming the article ofFIGURE 2, such a helix, while in the green state, is inserted into asnug fitting cylinder of extruded or pressed green alumina. Firing isthen accomplished using the aforementioned firing temperature at 1650C., but with a soak period preferably about one-half hour at the maximumtemperature, followed by cooling to room temperature over about a 1 to 4hour period. Should a catalytic oxidizer element for exhaust gases bedesired, the structure, after firing, may be dipped briefly in a watersolution of dilute (about 1%) palladium chloride (or platinum chloride,chromic acid, etc.) and dried free of water at room temperature,followed by a brief firing up to 800 C. suitably in an air, without anyextended soak period at 800 C., using about a 4 hour cycle for thefiring.

Example 3 The basic raw material mix of Example 2 is coated out anddried to form a solvent-free plasticized film of about 20 mils thick.Using the conditions set forth in Example 2, the film is corrugated toprovide about 10 uniform-wave corrugations of about mils amplitude perinch. The corrugated film is cut into rectangular sections and thecorrugated sections placed between separating 20-mil thick plasticizedsheet mmebers formed of the same basic raw material as the corrugatedsections. Simultaneously the outer ridges on each side of the corrugatedsections are heat-bonded to the separating sheet members usingconditions as described in Example 2. The resulting structure is thatillustrated in FIGURE 4, but with a greater number of corrugated andfiat sheet members in the stack. Using a jewelers handsaw, the structureis cut so as to form the curved over-all configuration of a turbineblade (see FIGURE 3). About this structure, which constitutes theinternal reinforcement for the turbine blade, is then wrapped flexibleplasticized pre-cut green flat skin panels about 20 mils thick andsuitably formed of the same basic raw material as the corrugatedsections. The flat skin panels are glue-bonded to the edges of theinternal corrugated reinforcing sections using conditions as set forthin Example 2. Thereafter, the structure is fired by heating it to about1650 C. over a 12 hour firing cycle with the maximum soak temperaturebeing maintained for about one-half hour. After firing, the turbineblade can withstand considerably more thermal shock without cracking orrupture than solid blades of the same material, and can be used inextraordinary intense heat environments (e.g., red-heat") withoutcorrosion or noticeable deterioration.

Example 4 A basic plastic raw material mix is prepared consisting of 80parts by weight of beryllia having a particle size averaging about 1micron, 20 parts of tetrapolymer of Example 2, and about 50 parts byweight of the solvent mixture of Example 2. About 1 part by weight ofconcentrated nitric acid solution is added as a defiocculating agent tothe basic new material mix. The nitric acid serves to neutralize anyresidual alkaline beryllium compounds in the beryllia, and preventscoagulation in the tetrapolymer slurry. It has been found that alkalineceramic particles should be neutralized to prevent coagulation in aslurry formed using this tetrapolymer.

The slurry is then ball milled for about 8 hours so as to gain a uniformblend. The blended slurry is coated at a thickness of about 10 mils on alow adhesion surface, i.e., polyethylene coated paper, and the coatingallowed to partially dry (e.g., to dry until only about 20% of thesolvent remains in the film). A second IO-mil thick layer consisting ofthe same raw material slurry as used for the first layer, but inaddition containing approximately parts of enriched uranium oxide fuel(the uranium being essentially U-235), is then knife-coated over thepartially dried first layer and also allowed to partially dry (i.e., toabout 20% solvent retention). Finally, a third layer is knife-coatedover the second layer, the third layer also being mils thick andconsisting solely of the same basic raw material mix as used for thefirst layer. The entire laminate is dried in air at room temperature,the final dried thickness being on the order of about to mils.

The dried laminate is lifted from the polyethylene coated paper andabout half of the laminated strip placed u on a 10-mil aluminum foil.The aluminum foil and the laminate are passed together through acorrugating apparatus, the rolls of which are about 270 F. and provideabout 8 corrugations of mils amplitude per inch.

The portion of the laminate of this example not corrugated is painted orsprayed on one side with a thin coating of a slurry consisting of thebasic raw material mix of this example, and then placed with its coatedside against the exposed ridges of the corrugations of the corrugatedfilm. The solvent from the coated slurry penetrates slightly into thesurface portions of the ridges of the contacting corrugations and thenevaporates at room temperature. Squares are cut from the resultingassembly sheet consisting of corrugated film and fiat sheet material,and the squares stacked to form the article illustrated in FIGURE 4,with the exopsed portions of the fiat sheet material of each assemblysheet being painted or sprayed with a thin coating of the basic rawmaterial mix of this example so as to glue the composite structuretogether at points of contact between the ridges of corrugations and thefiat sheet members. Exposed edges of the sandwich sheet members are alsocoated with the basic raw material mix to seal in the fuel. Thecomposite article resulting is then allowed to dry at room temperatureand fired in an inert atmosphere (e.g., a hydrogen atmosphere) using a16 hour firing cycle and a maximum soak temperature of about 1550 C.maintained for /2 hour in the middle of the firing cycle. The resultingcomposite sintered ceramic structure is useful as a fuel element inatomic reactors.

It will be understood that a wide variety of sinterable materials may befabricated into corrugated shapes as taught herein, and that theforegoing examples are illustrative of but a few specific materialswhich may be employed. Further illustrations of refractory materialswhich may be used are zirconia, cordierite, fosterite, zircon, bariumtitanate, porcelain, thoria, urania, steatite, magnesia, Samaria,gadolinia, various carbides including boron carbide, spinels, etc. Metalpowders may be shaped into corrugated intricate structures and fired inoxidizing atmospheres to create the refractory ceramic articles of theinvention, or carbide articles may be formed by fabricating preforms ofcorrugated articles using reactive ingredients which on firing produce afinal refractory article as illustrated. Articles as herein taught maybe formed from sinterable ceramic and metal mixtures, e.g., chromiurnand alumina mixtures, to form cermets. Sinterable ceramic particles orfibers of a refractory nature, together with materials serving asfillers or reinforcing media, may be employed to form theheat-rigidified articles hereof. Metal rods, wire screens, etc. may beused for reinforcement of ceramic sheets corrugated as taught herein.Indeed, fillers or reinforcing materials in the form of rods or variousshapes formed of sinterable metal particles held together by hinder orplasticizing ingredient as here in discussed, may be dispersedthroughout a mixture of plasticized ceramic particles (the metalparticles and ceramic particles being preferably so selected for approximately matching coetficients of thermal expansion) and the compositeformed into an article of complex configuration with corrugated membersas described, to give upon firing (preferably in an inert atmospheresuch as argon or hydrogen) an end article of improved tensile strengthand resistance to cracking on thermal shock. A particular advantagegained using this type of reinforcing filler media as distinguished fromusing pre-formed fully densified solid non-sinterable wires of metal(e.g., conventional metal wires, screens, etc.) is that of avoidingstructural defects such as holes, cracks, protruding ends of wire andthe like in a final corrugated end article of the invention, whichresults from the ceramic shrinking on sintering while the already densemetal filler does not.

As illustrated in Example 4, sandwich laminates of various sinterableparticles may be corrugated to form the articles hereof; and thelaminate may if desired be formed of alternate layers of ceramic andmetal sinterable particles, with the plasticizing binder for various ofthe layers being the same or different. For example, a laminate of alayer of zirconium oxide powder, a layer of molybdenum powder, and alayer of zirconium oxide powder, each layer being plasticized and bondedto the others 'by the binder of Example 2, may be corrugated and thenformed into a honeycomb structure or other structural configurationillustrated in the drawing using a slip of zirconium oxide-binder fortemporary gluing or bonding between meeting ridges of corrugations ofdifferent corrugated layers (in the case of a honeycomb) or betweenmeeting lines of junction between fiat plates and corrugated sheets (asillustrated in the drawings), to form, upon firing in an inertatmosphere such as argon or hydrogen, a complex sintered shape ofelectrically conductive materials protected on its multitudinoussurfaces by a heat and corrosion resistant refractory ceramic.

The corrugated complex thin-walled structural configurations hereof mayalso be formed with additions of mica (which may indeed serve as abinder material). Non-refractory materials such as, for example, alkalicompounds and the like may be incorporated in small amounts (e.g., a fewpercent) as fluxes in the sinterable films of this invention, ifdesired. For example, certain complex mixtures of lithium aluminumsilicates are useful to form bodies of low or zero coefficient ofthermal expansion. Refractory fluxes such as alkaline earth compounds(e.g., oxides, fluorides, nitrates, etc.) may also be useful.

While the greatest benefits of the invention appear to arise in caseswhere sinterable refractory ceramic materials are employed, it is alsotrue that the principles of this invention may be used to advantage toform corrugated articles out of non-ductile, non-malleable, butsinterable metals such as, for example, brittle powdered tungsten,beryllium, molybdenum, tantalum, intermetallic compounds, such as, forexample, zirconium diboride. Thermal or other treatments may be employedto alter end-properties of articles formed of metals, as

for example, to introduce some flexibility or ductility. Unified complexstructures having corrugated members of ceramic and metal membersinterspersed are possible. The process herein taught for formingcorrugated heat-resistant articles as well as sinter-welding is alsouseful to form sinter-welded intricate corrugated structures out of moreconventional metals (e.g., iron, stainless steel, etc.), and wouldintroduce important economies and advantages in the production of suchcorrugated structures. Particularly significant in this respect is theability to form intricate corrugated metal structures of uniformcomposition and crystal structure (as observable in X-ray dilfractionanalysis) throughout the intricate structures, including all bondedareas and sheet portions, which has been impossible using metal brazingtechniques to join metals together as done in the past. If desired,these thermally rigidified metal structures may be subjected to varioustemperature or other treatments to alter end-properties of the article,as for example, mild oxidation to improve or increase rigidity.

A variety of organic plasticizing ingredients, e.g., polyvinyl chloride,phenol formaldehyde resins, nitrile rubbers, etc., and combinationsthereof may be employed in the formation of the green sinterable filmsand sheet members used to fabricate corrugated articles according tothis invention. Several illustrative well-known organic plasticizingingredients suitable for use as set forth in my patent application filedMay 7, 1957, Ser. No. 657,503; and the disclosure thereof is hereincorporated by reference. Also, water soluble plasticizing agents orbinders, e.g., methyl cellulose, may be used to form flexible filmshighly filled with sinterable particles; and the volatile temporaryliquid vehicle for slip or paste formation may be water. In general, theplasticizing agent selected for any particular sinterable particulatematerial will be so selected as to be non-reactive with the sinterableparticles of the mass during subsequent firing. However, while burn-outbinders non-reactive with the sinterable material are preferred, it issometimes satisfactory to employ binders which react with the sinterableparticles to form refractory masses in situ. Even inorganic plasticizingingredients or low temperature curing agents or binders, e.g., solublealkali silicates, bentonites, aluminum hydrogen phosphates, as well as avariety of others, may be employed where the presence of residualmaterial of the plasticizer or binder is not detrimental to otherproperties required in final corrugated end products.

It will be understood that many variants of the structural configurationspecifically illustrated in the drawings hereof (e.g., employment of acenter conduit member in the article of FIGURE 2, arcing of laminatedcorrugated structures, etc.) are possible without departing from theessential corrugated structural feature of the new articles of thisinvention, as further set forth in the claims appended hereto.

Many uses for the articles hereof in addition to those heretoforerecited will readily suggest themselves to those becoming familiar withthis invention. Some illustrative additional uses are the following:nuclear reactor components, filters, insulators (acoustical, electrical,and thermal), noise suppressors or mufilers, components of aircraft andmissiles, radomes, circuit bases, wave guides, combustion ports, rocketnozzles and vanes, a base support structure for ablation materials, heatexchangers for automobile afterburners (e.g., particularly cross-currentheat exchangers such as illustrated in FIGURE 1, or counter-current heatexchangers), etc.

That which is claimed is:

1. A fired ceramic structural article having a high surface area inrelation to its size, and comprising at least one ceramic sheet memberand at least one thin corrugated web of ceramic having an amplitude ofcorrugations at least as great as the thickness of said web, saidcorrugated web being no greater than 50 mils thick and being permanentlyceramically bonded to said sheet member in a non-separable manner alongat least a portion of some of the ridges on one side of said corrugatedweb so as to provide at least one set of aligned passages defined bygrooves of said corrugated web between adjacent ridges permanentlybonded to said sheet member, said article being formed by a processinvolving ceramically firing to rigidity a self-supporting green ceramicstructure of like configuration having firm temporary bonds of greenceramic corresponding in location to said permanent ceramic bonds insaid structural article, said green ceramic structure comprisingmaterials convertible into said fired ceramic structural article by saidceramic firing, and said firm temporary bonds of said green ceramicstructure being unified continuous material paths of green ceramicmaterial between parts of said green structure so temporarily bonded.

2. As an article of manufacture, a refractory nonductile, non-malleable,brittle structure having multiple surfaces and a high surface area inrelation to its size, consisting essentially of at least two refractorynonductile non-malleable brittle sheets less than 50 mils thick, atleast one of which has corrugations which have an amplitude at least asgreat as the thickness of said one sheet, said sheets being permanentlyinseparably united together by means of refractory non-ductilenonmalleable brittle material at a plurality of lines of contacttherebetween so as to define multiple aligned passages in saidstructure, said structure being self-supporting and non-flowing even attemperatures as high as 1000 C., and being formed by a process involvingthermal n'gidification of a green structure of like configuration havingfirm temporary bonds corresponding in location to the portions of saidsheets permanently inseparably united in said refractory structure, saidgreen structure comprising refractory sinterable particulate and beingconvertible into the material of said refractory structure by thermalrigidification, and said firm temporary bonds of said green structurebeing unified continuous material paths of refractory sinterableparticulate material between parts of said green structure sotemporarily bonded.

3. A heat-resistant multi-passaged thermally-rigidified brittlecartridge having a corrugated sheet member no greater than 50 mils thickpermanently united along ridges of several of its corrugations to anon-corrugated sheet member and having the two sheet members shaped inthe form of a helix, said cartridge being formed by winding thecorrugated sheet and non-corrugated sheet in said helical form andfirmly temporarily bonding said corrugated sheet along ridges of severalof its corrugations to said non-corrugated sheet while each said sheetcomprises sinterable inorganic particles in a flexible plasticized greenstate prior to thermal rigidification of the same, said firm temporarybonds being green unified continuous material paths of sinterableinorganic particles between parts of the green structure temporarilybonded, the strength of said temporary bonds being suflicient tomaintain said flexible plasticized green sheets together and preventrelative displacement thereof during handling of the structure prior tothermal rigidification of the same, and the composition of said firmtemporary bonds and said sheets in the flexible plasticized green statebeing converted into the composition of said thermally rigidifiedbrittle cartridge by said thermal rigidification.

4. A rigid sintered inorganic multiple-webbed honeycomb article havingmultiple surfaces and a high surface area in relation to its size,formed of a plurality of corrugated sheets less than about 50 mils thickin stacked relationship to one another with the ridged portions ofadjacent corrugated sheets in aligned abutting relationship andpermanently inseparably united together to form multiple passages in thearticle, each passage being defined by paired grooves of corrugationsbetween permanently inseparably united abutting ridges of corrugations,said article being formed by sintering a green article of likeconfiguration formed of green corrugated sheets firmly temporarilybonded in the green state along aligned abutting ridges of corrugations,said green article being composed of sinterable inorganic particlesconvertible into said rigid inorganic honeycomb article by sintering,and said firm temporary bonds being green unified continuous materialpaths of sinterable inorganic particles between parts of said greenstructure temporarily bonded, the strength of said temporary bonds beingsufficient to maintain said green corrugated sheets together and preventrelative displacement thereof during handling of said green articlepreliminary to sintering the same.

5. A rigid inorganic multiple-webbed thermally rigidified article ofpredetermined size and shape having multiple surfaces and a high surfacearea in relation to its size, formed from at least two sheets, at leastone of which is no greater than 50 mils thick and corrugated with anamplitude of corrugations at least as great as the thickness of thesheet, and at least one of which comprises a laminate having ceramic onthe outer exposed surface portions thereof, said sheets beingpermanently inseparably bonded together at a plurality of lines ofcontact therebetween so as to define a series of aligned passages insaid article, said article being self-supporting and formed by thermalrigidification of sinterable inorganic particles in a green structure oflike configuration having firm temporary bonds corresponding in locationto said permanent bonds in said article, said firm temporary bonds ofsaid green structure being green unified continuous material paths ofsinterable inorganic particles between parts of said green structuretemporarily bonded, the strength of said temporary bonds beingsufficient to maintain the firmly temporarily bonded parts of said greenstructure together and prevent relative displacement thereof duringhandling of said green structure preliminary to thermal rigidificationof the same.

6. A rigid inorganic multiple-webbed thermally-rigidified structureformed from at least two sheets, at least one of which is no greaterthan 50 mils thick and corrugated with an amplitude of corrugations atleast as great as the thickness of the sheet, and at least one of whichconsists essentially of ceramic with metal present therewith, saidsheets being permanently unified at a plurality of lines of contacttherebetween so as to define multiple passages in said structure, saidstructure being self-supporting and formed by a process involvingthermal rigidification of a green preform of like configuration havingfirm temporary bonds corresponding in location to said locations ofpermanent unification in said structure, said green preform comprisingsinterable inorganic particles convertible into the composition of saidrigid inorganic structure by thermal rigidification, and said firmtemporary bonds of said green preform being green unified continuousmaterial paths of sinterable inorganic particles between parts of saidgreen preform temporarily bonded, the strength of said temporary bondsbeing sufficient to maintain the firmly temporarily bonded parts of saidgreen preform together and prevent relative displacement thereof duringhandling of said green preform preliminary to thermal rigidification ofthe same.

7. A process for forming an intricately-shaped brittle inorganicthermally-unified article from plasticized shapes of sinterableparticles in such manner as to form permanent welds of juncture betweenabutting parts of said article, with the permanent welds being ofcomposition and crystal structure the same as the composition andcrystal structure of the surface layers of the abutting parts, saidprocess comprising coating upon the portion of liquid-free greenorganically plasticized shapes of sinterable particles to be joined afluid slip having substantially the same inorganic solids analysis asthe surface layer composition of the green shapes to be joined, thesolid ingredients of said slip being rendered fluid by the presence insaid slip of volatile normally-liquid material capable of penetratingsurface portions of said organically plasticized green shapes to bejoined, said coating being such as to cause said volatile normallyliquidmaterial in said slip to penetrate said surface portions of saidorganically-plasticized green shapes so coated, fabricating a preform ofthe desired end article by placing slip coated portions of said shapesin abutting relation to portions of the green plasticized shapes where abond of juncture is desired, drying the volatile normallyliquid materialfrom the slip coated areas of the preform, and then thermallyrigidifying the green preform.

8. A process for forming an intricately-shaped brittle inorganicthermally-unified article from plasticized shapes of sinterableparticles in such manner as to form permanent welds of juncture betweenabutting parts of said article, with the permanent weld being ofcomposition and crystal structure the same as the composition andcrystal structure of the surface layers of the abutting parts, saidprocess comprising forming green plasticized shapes consisting ofsinterable particles temporarily unified with an organic binder materialexhibiting a thermoplastic stage on heating, fabricating a preform ofthe desired end article by heat-sealing portions of said greenplasticized shapes to those portions of the green plasticized shapeswhere a permanent bond of juncture is desired in the end article, saidheat sealing being such as to form a temporary bond of unified greenplasticized material between those portions of said green plasticizedshapes where a permanent bond of juncture is desired in the end article,volatilizing the organic binder material from the green preform, andthen thermally rigidifying the preform article.

9. A heat-resistant brittle multiple-webbed helical article comprising athermally rigidified corrugated sheet member wound in overlappingconvolutions and permanently inseparably united along ridges ofcorrugations to portions of said helical article contacting said ridges,said sheet member being no greater than 50 mils thick and beingcorrugated with alternating ridges and grooves on each side thereof, theamplitude of corrugations being at least as great as the thickness ofsaid sheet member, and said article being made by thermally rigidifyinga green article formed by a process including corrugating a greenflexible sheet member of sinterable particles, winding said corrugatedsheet so as to form overlapping convolutions of the same, and firmlytemporarily bonding ridges of corrugations to portions of the greenarticle contacted by said ridges to maintain said firmly temporarilybonded parts together and prevent relative displacement thereof duringhandling of said green article prior to thermal rigidification of thesame, the firm temporary bonds of said green article being green unifiedcontinuous material paths of sinterable particles between parts of saidgreen article temporarily bonded.

10. A sintered metallic structural article having a multiple-webbedconfiguration with at least a portion of the webs thereof beingcorrugated and permanently sinterwelded to other webs of the articlealong ridges of said corrugations, said sinter-welds being ofsubstantially the same composition and crystalline structure as thesurface layer of a web so sinter-welded, said structural article beingformed by sintering a green structure of like configuration formed of aplurality of relatively flexible green webs of sinterable metalparticles, at least a portion of said webs being no greater than /8 inchthick and corrugated with alternating ridges and grooves of essentiallyuniform periodicity on each side thereof, with the amplitude ofcorrugations for each such web at least as great as the thickness of therespective web so corrugated, at least a portion of said corrugated websbeing firmly temporarily bonded along ridges of corrugations to othergreen webs of said green structure by green unified continuous materialpaths of sinterable metal particles between said ridges of corrugationsand other green webs of said green structure, the strength of said greenunified continuous material paths being sufi'icient to maintain saidfirmly temporarily bonded parts together and prevent relativedisplacement thereof during handling of said green structure, saidstructure being characterized by having a plurality of channels, themetal solids particulate of said temporary bonds being substantially thesame in composition as the metal solids particulate on the surface layerof a green web so temporarily bonded, and said temporary bonds beingconverted to said permanent sinter-welds during sintering said greenstructure.

11. A method for making a multiple-webbed ceramic article wherein thechannels are of predetermined shape and size which comprises forming agreen ceramic sheet out of mtaerials comprising pulverized ceramic andan organic binder therefor, corrugating the sheet, forming thecorrugated sheet into a green preform structure with at least a portionof the ridges of corrugations in said corrugated sheet abutting andfirmly temporarily bonded to green ceramic of said preform structure insuch manner as to form channels, said temporary bonds being accomplishedin the formation of said preform structure by volatilizing a volatileliquid vehicle included at least in the parts of said preform structureto be so temporarily bonded, and firing the green preform structure tosinter the particles into a unitary ceramic article, whereby thechannels in said green preform structure are maintained in said unitaryceramic article.

12. A ceramic article comprising a sleeve of ceramic and amultiple-webbed ceramic structure therewithin and permanentlysinter-bonded thereto, said multiple-webbed ceramic structure having asheet member no greater than 50 mils thick corrugated with an amplitudeof corrugations at least as great as the thickness of said sheet memberwith ridges of said corrugations permanently sinter-bonded to anothersheet member, said ceramic article being formed by sintering a greenceramic structural preform of like configuration having firm temporarybonds of unified green ceramic corresponding in location to saidpermanent sinter-bonds in said ceramic article.

13. A rigid inorganic multiple-webbed sintered article of predeterminedsize and shape having multiple surfaces and a high surface area inrelation to its size, formed from at least two sheets, at least one ofwhich is no greater than 50 mils thick and corrugated with an amplitudeof corrugations at least as great as the thickness of the sheet, and atleast one of which comprises a laminate having one type of ceramic onthe outer exposed surface portions thereof and a material of differentceramic composition embedded in a layer between said exposed surfaceportions, said sheets being permanently and inseparably sinter-bondedtogether at a plurality of lines of contact therebetween so as to definemultiple passages in said article, said article being self-supportingand formed by sintering a green ceramic structure of like configurationhaving firm temporary bonds of unified green ceramic corresponding inlocation to said permanent sinter-bonds in said article, said firmtemporary bonds being at least effective to prevent displacement of saidgreen ceramic sheets with respect to each other during handling of saidgreen ceramic structure.

14. A process of making a rigid structural element having multiplecorrugated components, comprising (1) forming a flexible green sheet nogreater than /s inch thick and consisting essentially of sinterableinorganic particles held togeher by a flexible binder material, (2)corrugating said sheet with a uniform and repetitive amplitude ofcorrugations at least as great as the thickness of said sheet, (3)temporarily bonding the ridges of corrugations of said sheet to anotherflexible green sheet no greater than Vs inch thick and consistingessentially of sinterable inorganic particles held together by aflexible binder material, thereby to form an intermediate assemblyconsisting of a corrugated green sheet temporarily bonded along ridgesof corrugations to another green sheet, (4)

stacking a plurality of intermediate assemblies while simultaneouslytemporarily bonding the exposed ridges of corrugations of saidintermediate assemblies to contacting portions of adjacent assemblies inthe stack, thereby to form a multiple passaged green structural elementhaving a high exposed surface area in relation to its size, saidtemporary bonds being green unified continuous material paths ofsinterable inorganic particles, the strength of said temporary bondsbeing sufiicient to maintain the temporarily bonded parts togetherduring handling prior to firing, and (5) firing said green structuralelement to a rigid state, whereby the temporary bonds of said greenstructural element are converted to permanent inseparable rigid weldedjoints in said rigid structural element.

15. A refractory structure having a high surface area in relation to itssize, comprising at least two thin selfsupporting refractory rigidsheets, at least one which is no greater than 50 mils thick andcorrugated with corrugations of an amplitude at least as great as thethickness of said sheet, said corrugated sheet being permanentlyrefractorily bonded in an inseparable manner to the second sheet alongat least a portion of some of its ridges of corrugations such that aseries of aligned passages are formed in said structure, and saidstructure being formed by thermally uniting non-metallic inorganicparticles in a green structural preform of like configuration byinfiltration with molten metal, said green structural preform beingcharacterized by having firm temporary bonds of green unified continuousmaterial paths of non-metallic inorganic particles between ridges ofcorrugations of the corrugated sheet and the second sheet.

16. A process for forming an intricately-shaped sintered metal articlefrom plasticized green shapes of sinterable metal particles, the processbeing such as to form permanent sinter-welds of juncture between partsof said article, said process comprising coating upon the portion ofliquid-free green organically-plasticized shapes of sinterable metalparticles to be joined a fluid slip containing volatile normally-liquidmaterial capable of penetrating surface portions of saidorganically-plasticized green shapes to be joined, said coating beingsuch as to cause said volatile normally-liquid material to penetratesurface portions of said organically-plasticized green shapes so coated,fabricating a preform of the desired end article by placing coatedportions of said shapes in abutting relation to portions of the greenplasticized shapes where a bond of juncture is desired, drying thevolatile normally-liquid material from the coated areas of the preform,and then sintering the green metal preform.

17. A process for forming an intricately-shaped sintered metal articlefrom plasticized green shapes of sinterable metal particles, the processbeing such as to form permanent sinter-welds of juncture between partsof said article, said permanent sinter-welds being of a composition andcrystal structure the same as the composition and crystal structure ofthe surface layers of parts of said article so joined, said processcomprising coating upon the portion of liquid-free greenorganically-plasticized shapes of sinterable metal particles to bejoined a fluid slip having substantially the same metal solids analysisas the surface layer composition of the green shapes to be joined, thesolid metal ingredients of said slip being rendered fluid by thepresence in said slip of volatile normally-liquid material capable ofpenetrating surface portions of said organically-plasticized greenshapes to be joined, said coating being such as to cause said volatilenormally-liquid material in said slip to penetrate surface portions ofsaid organically-plasticized green shapes so coated, fabricating apreform of the desired end article by placing slip coated portions ofsaid shapes in abutting relation to portions of the green plasticizedshapes where a bond of juncture is desired, drying the volatilenormally-liquid material from the slip coated areas of the preform, andthen sintering the green metal preform.

18. A rigid inorganic multiple-webbed sintered structure comprising aplurality of sintered inorganic sheets arranged in superimposedrelationship, at least the alternate sheets of said structure being nogreater than A; inch thick and being corrugated with ridges and groovesof essentially uniform periodicity on each side thereof, with theamplitude of corrugations in each said respective corrugated sheet beingat least as great as the thickness of said respective corrugated sheet,said corrugated sheets being permanently and inseparably bonded alongthe contact between ridges of the corrugations thereof to adjacentsheets in said structure by means of sintered inorganic material, thecomposition of said sintered inorganic material in said bonds beingessentially the same as the composition of the surface layer of saidsintered inorganic corrugated sheets adjacent said bonds, said rigidinorganic structure being self-supporting and formed by a processinvolving sintering a green preform of like configuration having firmtemporary bonds corresponding in location to the inseparable bonds insaid rigid inorganic structure,

said green preform comprising sinterable inorganic particles convertibleinto the composition of said rigid inorganic structure by sintering,

said firm temporary bonds of said green preform being green unifiedcontinuous material paths of sinterable inorganic particles between saidparts so temporarily bonded,

the strength of said temporary bonds being sufiicient to maintain thefirmly temporarily bonded parts of said green preform together andprevent relative displacement thereof during handling of said greenpreform preliminary to sintering the same.

19. The structure of claim 18 characterized by having essentially thesame composition in all portions thereof.

20. The structure of claim 18 consisting essentially of metal.

21. The structure of claim 18 consisting essentially of ceramic.

22. A fired ceramic structural article having a high surface area inrelation to its size, and comprising at least one ceramic sheet memberand at least one thin corrugated web of ceramic having an amplitude ofcorrugw tions at least as great as the thickness of said web, saidcorrugated web being no greater than 50 mils thick and being permanentlyceramically sinter-welded to said sheet member in a nonseparable manneralong at least a portion of some of the ridges on one side of saidcorrugated web so as to provide at least one set of aligned passagesdefined by grooves of said corrugated web between adjacent ridgespermanently sinter-welded to said sheeet member.

23. A fired ceramic structural article having a high surface area inrelation to its size, and comprising at least one ceramic sheet memberand at least one thin corrugated web of ceramic having an amplitude ofcorrugations at least as great as the thickness of said web, saidcorrugated web being no greater than 50 mils thick and beingperm'anently ceramically bonded to said sheet member in a non-separablemanner along at least a portion of some of the ridges on one side ofsaid corrugated web so as to provide at least one set of alignedpassages defined by grooves of said corrugated web between adjacentridges permanently bonded to said sheet member, said article beingformed by a process involving ceramically firing to rigidity aselfsupporting green ceramic structure of like configuration having firmtemporary bonds corresponding in location to said permanent ceramicbonds in said structural article, said green ceramic structurecomprising materials canvertible into said fired ceramic structurearticle by said ceramic firing, and said firm temporary bonds of saidgreen ceramic structure joining parts of said green structure sotemporarily bonded.

24. As an article of manufacture, a refractory nonductile, nonmalleable,brittle structure having multiple surfaces and a high surface area inrelation to its size, consisting essentially of at least two refractorynonductile nonmaileable brittle sheets less than 50 mils thick, at leastone of which has corrugations which have an amplitude at least as greatas the thickness of said one sheet, said sheets being permanentlyinseparably united together by means of refractory nonductilenonmalleable brittle material at a plurality of lines of contacttherebetween essentially free of any demarcation line so as to definemultiple aligned passages in said structure, said structure beingself-supporting and nonflowing even at temperatures as high as 1000 C.

25. As an article of manufacture, a refractory nonductile, nonmalleable,brittle structure hvaing multiple surfaces and a high surface area inrelation to its size, consisting essentially of at least two refractorynonductile nonmalleable brittle sheets less than 50 mils thick, at leastone of which has corrugations which. have an amplitude at least as greatas the thickness of said one sheet, said sheets being permanentlyinseparably united together by means of refractory nonductilenonmalleable brittle material at a plurality of lines of contacttlierebetween so as to define multiple aligned passages in saidstructure, said structure being self-supporting and nonflowing even attemperatures as high as I000 C., and being formed by a process involvingthermal rigidification of a green structure of like configuration havingfirm temporary bonds corresponding in location to the portions of saidsheets permanently inseparably united in said refractory structure, saidgreen structure comprising refractory sinterable particulate and beingconvertible into the material of said refractory structure by thermalrigidification, and said firm temporary bonds of said green structurejoining parts of said green structure so temporarily bonded.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,288,061 6/1942 Arnold 165-166 2,506,244 5/1950Stopka 165-10 2,552,937 5/1951 Cohen 25-156 2,887,456 5/ 1959 Halford eta1 252-477 2,900,254 8/1959 Raiklen -214 2,875,501 3/1959 Graveley264-59 2,977,265 3/1961 Forsberg et al. 154-43 3,112,184 11/ 1963Hollenbach 264-59 2,224,810 10/ 1940 Cumfer 154-286 2,251,066 7/1941Persson et al. -167 2,526,657 10/1950 Guyer 252-477 X 2,566,735 4/1951Lepie 154-99 2,636,825 4/ 1953 Nicholson 106-44 2,703,921 3/1955 Brown165-141 X 2,730,434 1/1956 Houdry 252-477 X 2,814,857 3/1957 Duckworth106-44 2,992,981 7/1961 Thomson et al. 60-108 FOREIGN PATENTS 750,303 6/1956 Great Britain.

CHARLES SUKALO, lPrimary Examiner

